1. Field of the Invention
This invention relates generally to deceleration control systems for vehicles, and more particularly concerns an adaptive brake application and initial skid detection system for braking of one or more wheels of an aircraft during landing that prevents deep initial skids, and to thus allow rapid brake application in a controlled manner.
2. Description of the Related Art
Anti-skid and automatic braking systems commonly have been provided on commercial and large turbine aircraft to aid the deceleration of the aircraft upon landing. Modern anti-skid systems typically optimize braking efficiency by adapting to runway conditions and other factors affecting braking to maximize deceleration, corresponding to the level of brake pressure selected by the pilot. In conventional antiskid systems, brakes are typically applied mechanically via a metering valve by the pilot, and as soon as the wheel brake pressure approaches the skid level, such as when an initial skid is detected, a brake pressure value is used to initialize the antiskid control system. However, it has been found that the success of this method can be affected by such factors as the mode of aircraft operation, aircraft weight, tire/runway interfaces, and the like. It would therefore be desirable to provide an adaptive brake application system that can adjust brake pressure or torque application to account for such factors.
Furthermore, rapid pedal application by an aircraft pilot also can often create deep initial skids before an effective antiskidding brake pressure or brake torque is determined and skidding is effectively controlled by conventional antiskid and brake control systems. Eliminating or reducing initial skids would result in shorter aircraft stopping distances, which allow the aircraft to land on shorter runways, and can result in reduced tire wear. It would thus be desirable to provide an initial skid detection system to automatically anticipate initial skid conditions and adjust to prevent deep initial skids, to allow the pilot to depress the brake pedals at any rate, while still providing for rapid brake application in a controlled manner. The present invention provides an adaptive brake application and initial skid detection system that meets these needs.
Briefly, and in general terms, the present invention provides for an adaptive brake application and initial skid detection system that allows rapid brake application, while preventing deep initial skids, by implementation of a skid anticipation system that is initialized as soon as a wheel approaches a skid level to reduce brake application pressure or torque and to apply brakes in a controlled manner.
The invention accordingly provides for a xe2x80x9csmartxe2x80x9d brake application and initial skid detection system for braking of a wheel of an aircraft during landing. The system is applicable to one or more wheels having a wheel brake for applying brake torque to the wheel. A brake torque sensor generates brake torque signals that are a function of the braking torque applied to the wheel brake, and the brake torque signals are compared with a predetermined threshold brake torque. A wheel speed transducer produces wheel speed signals that are a function of the rotational speed of the wheel, and a wheel velocity signal is generated based upon the wheel speed signals. The wheel velocity is compared with a reference velocity signal for generating wheel velocity error signals indicative of the difference between the aircraft wheel velocity signals and the reference velocity signal. A torque bias modulator integrator is also provided that is responsive to brake torque signals for adjusting the wheel velocity error signals to provide an anti-skid control signal, and in one currently preferred embodiment the torque bias modulator integrator is initialized with the predetermined threshold brake torque plus a predetermined constant torque value. A command processor generates a command brake torque signal generated in response to a deceleration command, and brake torque comparison means are provided for comparing the brake torque signals with the command brake torque signal for generating brake torque difference signals indicative of the difference between the brake torque signals and the command brake torque signal. Control means provide an adjusted brake torque signal to the wheel brake to control the wheel brake independently of operator brake application, in response to the brake torque difference signals. In another presently preferred embodiment, the torque bias modulator integrator is initialized to the value of a measured brake torque when the wheel velocity error signal indicates the beginning of a skid.
In a currently preferred embodiment, means are also provided for adjusting the brake torque error signals by a proportional torque gain, an integral torque gain, and a differential torque gain. In another presently preferred embodiment, transient control means for providing a proportional control signal and compensation network means, both responsive to the velocity error signal, are also provided, and the outputs of the transient control means and compensation network means are summed with the output of the torque bias modulator integrator.
From the above, it can be seen that the present invention provides a system and method to initiate brake control after rapid application of the brake pedal, but before the onset of skidding occurs. These and other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings, which illustrate by way of example the features of the invention.